The best place to look for questions about a specific tool is to click on the "?" icon at the top left of PD's main window to bring up the online Help site. You can also do that by hitting the F1 key.

I hadn't noticed that announcement so I typed in object selection in the Search field and saw a hit for Latest Updates, which led to this page on creating custom masks in the Brush Designer.

Please see the important note about the hardware and software version requirements highlighted in yellow just before item 4., as your system must meet all requirements for the AI object selection feature to be available.